The major objective is to elucidate the role of diet related calorigenesis and related areas of carbohydrate and lipid metabolism during the early phases of obese development. Three varieties of genetically obese rodents are used: the obese hyperglycemic mouse (ob/ob), the yellow obese mouse (Ay/a), and the fatty rat (fa/fa). Energy expenditure in tissues will be evaluated at the cellular and subcellular levels to ascertain which tissues reflect the lower oxygen consumption exhibited by the intact animals. The effect of dietary fat and carbohydrate and physical activity, introduced immediately after weaning, on the development of energy expenditure at the gross, cellular, and subcellular levels will be evaluated. Mitochondrial respiration will be studied as well as activities of selected mitochondrial enzymes in the course of these experiments. The development of insulin resistance by tissues of obese rodents may contribute to impaired energy metabolism. Therefore the action of insulin on glucose transport and oxidation will be studied. The results of the proposed investigations should increase our understanding of the role that early dietary experiences may have on the development of energy metabolism, body composition and adiposity.